High bandwidth, high efficiency dc-dc multilevel converter topology

ABSTRACT

A multilevel DC-DC converter includes a voltage source that provides a voltage Vout 1  to at least one charge converter circuit and an output filter capacitor having an associated output voltage Vout 2 . The at least one charge converter circuit includes a transformer having at least one primary winding and at least two secondary windings, a primary and secondary circuit each having at least two switching elements, and a control unit which receives a control signal, such as but not limited to an envelope tracking signal, which represents a desired output voltage. The control unit is arranged to provide output control signals to the respective switching elements of the primary and secondary circuits to activate and deactivate the respective switching elements to obtain a desired output voltage Vout 2 . The multilevel DC-DC converter can be arranged to operate as a boost converter or as a buck-boost converter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to DC-DC converters and, moreparticularly, to multilevel topologies for such converters.

2. Description of the Related Art

DC-DC converters are a class of power converter. They are used toconvert a direct current (DC) signal from one voltage level to another.These converters are commonly used in portable electronic devices thatare powered by batteries, such as laptops and cellular phones. DC-DCconverters are particularly useful in applications that have severaldifferent sub-systems, requiring several different voltage input levels.

There are several different schemes for DC-DC conversion. Linearregulators convert an input voltage to a lower output voltage bydissipating power through thermal radiation. For larger voltage drophigh-current applications, these devices are inefficient and, thus,rarely used. A more commonly used scheme is switched-mode conversion.Switch-mode converters convert voltages by periodically storing energyin inductive and/or capacitive components and then releasing that energyto produce the desired voltage level. Inductive components store energyin the form of a magnetic field, whereas capacitive components storeenergy in an electric field.

DC-DC converters that use a magnetic energy storage mechanism employinductors or transformers. The output voltage is controlled bymodulating the duty cycle of the voltages used to charge the inductivecomponent. Common types of magnetic storage DC-DC converters includebuck and boost converters.

FIG. 1 a and FIG. 1 b are circuit diagrams of a typical boost converter10. Energy is periodically stored in an inductor L and then released tothe load. During each periodic cycle, a switch S is used to allowcurrent to flow through the inductor. When the switch is closed, currentflows through the inductor and stores energy from the current in amagnetic field. During this time, the switch acts like a short circuitin parallel with the diode and the load, so no inductor current flows tothe load. When the switch is opened, the short circuit is removed andinductor current is allowed to flow through the load; this increases theimpedance of the circuit, which requires either a decrease in current oran increase in voltage to maintain a constant output voltage. Theinductor will tend to resist such a sudden change in the current, whichit does by acting as a voltage source in series with the input source,thus increasing the total voltage seen by the load and therebypreserving (for a brief moment) the current level that was seen when theswitch was closed. This is done using the energy stored by the inductor.Over time, the energy stored in the inductor will discharge into theload, bringing the net voltage back down. If the switch is cycled fastenough, the inductor will not discharge fully in between chargingstages, and the load will always see a voltage greater than that of theinput source alone when the switch is opened.

FIG. 2 a and FIG. 2 b are circuit diagrams of a typical buck converter20. Energy is periodically stored in an inductor L and then released tothe load. During each periodic cycle, two switches SW_(H) and SW_(L) areused to alternately connect one end of inductor L to input source V_(IN)during the charge phase and to ground during the discharge phase. Whenthe high side switch SW_(H) is closed (shown in FIG. 2 a), currentthrough the inductor L (I_(L)) rises linearly, charging the inductor L.Then SW_(H) is opened and the low side switch SW_(L) is closed (shown inFIG. 2 b), and I_(L) decreases linearly, discharging the inductor intothe load. As the inductor L is discharging, I_(L) decreases but stillflows in the same direction into the load because the stored magneticenergy prevents the current through the inductor from changing directioninstantaneously. The switches are turned on and off periodically at afixed frequency such that the duty cycle determines the ratio of outputvoltage to input voltage. If the high side switch is opened before theinductor is fully charged, there will always be a voltage drop acrossthe inductor, such that the net voltage seen by the load will always beless than the input voltage source.

At least one challenge associated with boost and buck converters are areduced efficiency at high switching frequencies, as well as power loss.In some applications, for example wireless applications, in order toincrease the power efficiency, the converter providing the supplyvoltage can be modulated using “envelope tracking,” wherein theconverter is arranged such that its output voltage tracks an envelopesignal. However, converters using envelope tracking need to change theoutput voltage in short time periods, such as a matter of a fewnanoseconds. During the short time periods, the converters have tocharge and discharge filter capacitors in the range of severalmicrofarads. This fast charging/discharging calls for high frequency andhigh power converters, which can be bulky and inefficient.

SUMMARY OF THE INVENTION

A multilevel DC-DC converter is presented which overcomes the problemsnoted above. A voltage source provides a voltage Vout1 to at least onecharge converter circuit. The at least one charge converter circuit canbe comprised of several elements, such as: a transformer having at leastone primary winding and at least two secondary windings; a primarycircuit having at least two primary switches; a secondary circuit havingat least two secondary switches, wherein the secondary windings arecoupled to an output filter capacitor; and a control unit arranged toreceive a control signal which represents a desired output voltage Vout2and to provide output control signals to the primary and secondaryswitches in order to activate and deactivate the switches to obtain thedesired output voltage Vout2 on the output filter capacitor, such thatthe voltage level of Vout2 is controlled by the control signal.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a circuit diagram of a boost converter that is known in theart with a Switch S in a closed-state.

FIG. 1 b is a circuit diagram of the boost converter of FIG. 1 a withthe Switch S in an open-state.

FIG. 2 a is a circuit diagram of a buck converter that is known in theart with a high side switch operating in an on-state.

FIG. 2 b is a circuit diagram of a buck converter of FIG. 2 a with a lowside switch operating in the on-state.

FIG. 3 is a block diagram of a multilevel DC-DC converter according toan embodiment of the invention.

FIG. 4 a is a circuit diagram of a multilevel DC-DC converter accordingto an embodiment of the invention.

FIG. 4 b is a timing diagram for the multilevel DC-DC converter of FIG.4 a.

FIG. 5 a is a circuit diagram of a multilevel DC-DC converter accordingto an embodiment of the invention.

FIG. 5 b is a timing diagram for the multilevel DC-DC converter of FIG.5 a.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention provide a multilevel DC-DC converter thatutilizes a control signal to provide a desired output voltage.

FIG. 3 shows a block diagram of a multilevel DC-DC converter 100according to an embodiment of the invention. A voltage source 102provides a voltage Vout1 at a node 103 that is fed into a chargeconverter circuit 104. The charge converter circuit 104 is adapted to beconnected to external components to produce a desired output voltageVout2 at an output load 108. A control unit 106 is arranged to receive acontrol signal 110, such as but not limited to an envelope trackingsignal, which represents a desired output voltage Vout2, whereby thecontrol unit provides output control signals 112 to the charge convertercircuit 104 such that the output control signals provide operationalinstructions to the charge converter circuit 104 to obtain the desiredoutput voltage Vout2. The voltage source 102 is arranged to provide avoltage source to the charge converter circuit 104. In one embodiment,the voltage source 102 can be a battery, while in other embodimentsvoltage source can be the output of another circuit, such as but notlimited to a galvanically non-isolated converter, a galvanicallyisolated converter, a forward converter, a buck converter, or anyconstant voltage source known in the art.

The charge converter circuit 104 comprises at least one switchingelement wherein the at least one switching element is adapted to beswitched on and off in response to the output control signals 112provided by the control unit 106. The control unit 106 is arranged tocontrol a switching period during which the at least one switchingelement is switched on and off to provide the desired output voltageVout2. The charge converter circuit 104 is governed by the control unit106 such that the charge converter circuit produces an output voltageVout2 that is substantially similar to the desired output voltagerepresented by the control signal 110.

FIG. 4 a shows a multilevel DC-DC converter 200 according to anembodiment of the invention. FIG. 4 a is meant to convey one embodimentof the general system of FIG. 3, such that the discussion below detailsthe operation of the multilevel DC-DC converter 200 with reference toone possible exemplary embodiment. Thus, it is understood that otherembodiments according to the claims are possible. In one embodiment ofthe invention, the multilevel DC-DC converter 200 comprises a voltagesource 102 that provides a voltage Vout1 at a node 210 that is fed intoa charge converter circuit, whereby the charge converter circuitprovides an output voltage Vout2 at an output load 108.

In the embodiment shown in FIG. 4 a, the charge converter circuitcomprises a transformer 212 including a primary winding 214 and twosecondary windings 216. The transformer 212 is configured to be acenter-tapped transformer, though other transformer types could also beused. The charge converter circuit further comprises a primary circuit220 and a secondary circuit 230, with the primary circuit 220 havingfour primary switches 201, 202, 203, 204, and the secondary circuit 230having two secondary switches 205, 206, wherein the primary andsecondary switches may be N-channel MOSFETs, as shown in FIG. 4 a.

The primary switches 201-204 are arranged in a bridge configuration withone end of the primary winding 214 coupled to the node between primaryswitches 201 and 202 and the other end of the primary winding 214coupled to the node between primary switches 203 and 204. A supplyvoltage Vsup 208 is provided to the primary circuit 220 which is used toinduce a voltage step Vs in secondary circuit 230 that is combined withVout1 to produce the desired output voltage Vout2.

The secondary circuit 230 has a first pair of secondary switches 205,206, each of which is connected to a respective one of the secondarywindings 216 between the first node 210 and the output load 108. Asshown in FIG. 4 a, the first pair of secondary switches 205, 206 arearranged parallel to each other wherein the voltage source 102 isconnected to the source of each of the secondary switches 205, 206 whilethe respective drains of the secondary switches are connected to arespective secondary winding 216 of the transformer 212. In otherembodiments, the first pair of secondary switches 205, 206 can bearranged such that the source of each of the secondary switches isconnected to a respective secondary winding 216, while the respectivedrains of the secondary switches are connected to the output load 108.The secondary switches are arranged such that the secondary circuitoperates as a synchronous rectifier, allowing the multilevel DC-DCconverter to operate in continuous conduction mode.

The gate of each of the primary switches 201-204 and secondary switches205, 206 are connected to the control unit 106, such that the outputcontrol signals 112 can be applied to the respective gates of theprimary switches 201-204 and the secondary switches 205, 206. The outputcontrol signals 112 cause the primary switches 201-204 and secondaryswitches 205, 206 to activate and deactivate over a time period that isdetermined by the control signal 110 received by the control unit 106.The four primary switches 201-204 are grouped into two pairs, with onepair being switches 201/204 and the second pair being switches 202/203.The pairs of primary switches operate in complementary fashion, suchthat when the first pair 201/204 is activated, the second pair ofswitches 202/203 are deactivated, and vice versa. A switch which isactivated conducts current, while a switch which is deactivated doesnot. For purposes of this discussion, t_(on) represents the time duringa period when the first pair of primary switches 201/204 are activatedand the second pair of primary switches 202/203 are deactivated, andt_(off) represents the time during the period when the first pair ofprimary switches 201/204 are deactivated and the second pair of primaryswitches 202/203 are activated. The control unit 106 is preferablyarranged to be an envelope tracking unit that receives an envelopetracking signal 110. However, the invention is not intended to belimited to an envelope tracking application. In other embodiments, thecontrol unit 106 can be a bi-directional converter or the like.

The secondary switches 205, 206 are activated and deactivated based onthe output control signals 112, in conjunction with the primary switches201-204. When the first pair of primary switches 201/204 is activated,the secondary switch 206 is activated and the secondary switch 205 isdeactivated. When the second pair of primary switches 202/203 isactivated, the secondary switch 205 is activated and the secondaryswitch 206 is deactivated. While the primary switches 201-204 and thesecondary switches 205, 206 are operating, the charge current convertercan inject charges into the output capacitor Cout1 to get a fast voltagerise, such that the multilevel DC-DC converter of FIG. 4 a operates as aboost converter. Voltage source 102 can be any voltage source, such as abuck or boost converter.

FIG. 4 b shows a timing diagram which illustrates the operation of themultilevel DC-DC converter of FIG. 4 a. The control signal 110represents the desired output voltage Vout2 that is to be provided bythe charge converter circuit. When the desired output voltage Vout2 isequal to the voltage Vout1 (prior to time t1) that is delivered to thecharge converter circuit (Vout1=Vout2) by the voltage source 102, allthe primary switches 201-204 are deactivated and the secondary switches205, 206 are activated so that the output current flows from the voltagesource 102 through the secondary windings 216 and into the filtercapacitor Cout1. In this configuration, the secondary windings 216generate magnetic fluxes in opposite directions. As such, the resultingmagnetic flux in the transformer 212 is canceled out and issubstantially zero. As a result, there is effectively no inductancebetween the voltage source 102 and the output filter capacitor Cout1.

When the output voltage Vout2 at the output filter capacitor Cout1 isrequired to increase, as indicated by the control signal 110 at time t1,the first pair of primary switches 201/204 are activated, the secondaryswitch 205 is deactivated, and the secondary switch 206 remainsactivated; or, if the secondary switch 206 is deactivated, then it is tobe activated along with the first pair of primary switches 201/204. Thesecondary windings 216 act as a voltage source connected between Cin1and Cout1, causing a very fast voltage offset on Cout1 such that themultilevel DC-DC converter 200 operates as a forward converter. As seenin FIG. 4 b, this causes output voltage Vout2 to increase to a valuegreater than that of voltage source Vout1. The voltage increase of theoutput voltage Vout2 at the capacitor Cout1 will increase by a voltagestep Vs according to:

Vs=Vsup*(M/N)  Eq. 1

where Vsup is the supply voltage 208 that is provided to the primarycircuit 220 and M/N is the turn ratio of the transformer 212. The levelof the voltage step Vs can be determined by either the level of thesupply voltage Vsup or by the turn ratio of the transformer. The supplyvoltage Vsup 208 is combined with Vout1 to produce the desired outputvoltage Vout2. As such, the multilevel DC-DC converter exhibits a stepgranularity based on the voltage step Vs, such that the step granularityis in the range of 0, Vs.

During the next period, the first pair of primary switches 201/204 andthe secondary switch 206 are deactivated and the second pair of primaryswitches 202/203 and the secondary switch 205 are activated. The outputvoltage Vout2 at the capacitor Cout1 is maintained at the increasedvalue due to the voltage step Vs. This process continues, with thenumber of periods determined by the control signal 110. The turn ratioof the transformer 212 is designed to deliver high current into thecapacitor Cout1. Since the high current for quickly charging thecapacitor Cout1 is delivered by the transformer 212 with a high turnratio, there is no need for the primary circuit to utilize high currentswitches. There are no direct capacitive charges and discharges betweenthe two capacitances, such that there are no charge/discharge losses inthe switches. As such, the multilevel DC-DC converter efficiency isincreased considerably. The transformer 212 can have many different turnratios, such as but not limited to 5:1 through 10:1, but other turnratio values are also possible.

When the control signal 110 indicates that the desired output voltageVout2 is to return to the value of the voltage source Vout1 102 (timet2), then the activated primary switches 201-204 will become deactivatedand the secondary switches 205, 206 will become or remain activated.

As shown in FIG. 4 b, at time t1, there is a delay in increasing theoutput voltage Vout2 by the voltage step Vs, and at time t2 there is adelay in decreasing the output voltage Vout2. These delays are due tothe primary switches 201-204 and the secondary switches 205, 206receiving the output control signals 112 from the control unit 106 andthen activating or deactivating the switches in response to the outputcontrol signals 112. For example, the primary and secondary switches201-206 are shown as N-channel MOSFETs in FIG. 4 a, and an N-channelMOSFET is activated when the voltage of the output control signal 112 onthe gate exceeds the MOSFET's threshold voltage. When the gate receivesa command to activate the MOSFET switch, it takes some time for the gatevoltage to exceed the threshold voltage and thereby activate the MOSFETswitch. Furthermore, when an activated MOSFET switch receives a commandto deactivate the switch, the voltage at the gate takes some time toreduce to 0 volts, or below the threshold voltage. As such, there can bea time delay between a command to activate and/or deactivate the primaryand secondary switches 201-206. In order to reduce the time delay, thegate of each switch can be pre-biased, such that the switch is providedwith a deactivation voltage that is slightly below the threshold voltageof the switch instead of 0 volts, such that once an activation commandis received by the switch, the switch already has a voltage at the gatethat is close to the threshold voltage and thus less time is required toraise the voltage at the gate above the threshold voltage and therebyactivate the switch. This serves to improve the bandwidth of themultilevel DC-DC converter.

The multilevel DC-DC converter of FIGS. 4 a and 4 b is configured to bea two-level DC-DC boost converter. The two-level DC-DC boost converteris able to increase the output voltage by one voltage step Vs, such thatthe converter can generate the desired output voltage Vout2 to be equalto voltage Vout1 or voltage Vout1+voltage step Vs.

The embodiment of the multilevel DC-DC converter shown in FIG. 5 a isconfigured to be a three-level DC-DC buck-boost converter. Thethree-level DC-DC converter of FIG. 5 a is configured similarly to thetwo-level DC-DC converter of FIG. 4 a. However, the secondary circuit310 of FIG. 5 a is comprised of four secondary switches 205-208, forminga first pair of secondary switches 205/206 and a second pair ofsecondary switches 207/208, wherein the second pair of secondaryswitches is connected to the secondary windings 216 opposite the firstpair. For the same or similar elements or features, the same referencenumbers will be used throughout the application.

The inclusion of the second pair of secondary switches 207, 208 allowsthe multilevel DC-DC converter of FIG. 5 a to operate as a buck-boostconverter. The voltage step Vs is combined with the voltage Vout1 toproduce the desired output voltage Vout2 at the capacitor Cout1. Forexample, the voltage step Vs can be added to increase the output voltageVout2 by a voltage step Vs, which is calculated according to Eq. 1discussed above. In other instances, output voltage Vout2 can be reducedby the voltage step Vs such that the output voltage Vout2 is thedifference between the voltage Vout1 and the voltage step Vs.

FIG. 5 b shows a timing diagram for the converter of FIG. 5 a duringoperation. The control signal 110 represents the desired output voltageVout2 and provides instructions to the primary switches 201-204 and thesecondary switches 205-208. When the output voltage Vout2 is set to beequal to the voltage Vout1 that is delivered to the charge convertercircuit (Vout1=Vout2) by the voltage source 102 (prior to time t1), allthe primary switches 201-204 are deactivated and all the secondaryswitches 205-208 are activated. In this configuration, the outputcurrent flows from the voltage source 102 through secondary windings 216and into the filter capacitor Cout1, similarly as described above.

When the output voltage Vout2 is set to increase (time t1), the firstpair of primary switches 201/204 are activated, while secondary switch205 is deactivated and secondary switches 206-208 are activated orremain activated. As seen in FIG. 5 b, the output voltage Vout2 rises toa value greater than the voltage source Vout1 by a voltage step Vs.During the next period, the first pair of primary switches 201/204 andthe secondary switch 206 are deactivated and the second pair of primaryswitches 202/203 and the secondary switch 205 are activated, while thesecond pair of secondary switches 207, 208 remain activated. Thisprocess continues, with the number of periods determined by the controlsignal 110. In this configuration, the converter of FIG. 5 a, 5 b isfunctioning as a boost converter. When in boost mode, the secondaryswitches 207 and 208 must remain activated.

In order for the converter of FIG. 5 a, 5 b to operate in buck mode, thecontrol signal 110 provides instructions to the primary switches 201-204and secondary switches 205-208 so that the voltage of the voltage sourceVout1 is reduced by the voltage step Vs in order to provide the reducedoutput voltage Vout2. In such a configuration, the first pair ofsecondary switches 205/206 must remain activated while the second pairof secondary switches 207/208 are arranged to toggle between activatedand deactivated. For example, at time t5 of FIG. 5 b, primary switches202/203 and secondary switch 208 are activated, while secondary switch207 is deactivated. At this point, output voltage Vout2 is less thanvoltage Vout1 by the voltage step Vs. In the next period, primaryswitches 201/204 and secondary switch 207 are activated, while primaryswitches 202/203 and secondary switch 208 are deactivated; the firstpair of secondary switches 205/206 remain activated and the outputvoltage Vout2 remains at the reduced value. This process continues foras many periods as indicated by the control signal 110.

In order for the output voltage Vout2 to be less than the voltage Vout1by the voltage step Vs, the charge converter circuit discharges theoutput capacitor Cout1 and transfers the energy into the primary circuit212. The primary circuit 212 can store this energy in the form of amagnetic field due to the primary winding 214 of the transformer 212.The voltage step Vs is calculated similarly as discussed above and is afunction of the supply voltage Vsup provided to the primary circuit andthe turn ratio of the transformer 212. The step granularity of themultilayer DC-DC converter of FIG. 5 a has a range of −Vs, 0, Vs. Thetiming diagram in FIG. 5 b also shows that the converter exhibits a timedelay in increasing or decreasing the output voltage Vout2, similar tothe converter of FIG. 4 a. As such, the primary switches 201-204 andsecondary switches 205-208 can be pre-biased, similarly as discussedabove, to reduce the time delay.

It must be noted that the circuits illustrated herein are merelyexemplary. The order of the circuit elements in the multilevel convertercan be modified and still achieve the same result. It should be alsonoted that there may be additional circuits in the multilevel converterin addition to those discussed herein. For example, the charge convertercircuit can be comprised of a plurality of converter circuits such as afirst charge converter circuit and a second charge converter circuitarranged in a cascade configuration, wherein either the first or secondcharge converter circuit can be a boost converter or a buck-boostconverter. In such a cascaded configuration, the first converter circuitcan be adapted to increase or decrease its output voltage by a firstvoltage step Vs1=Vsup*M1/N1, where Vsup is the supply voltage and M1/N1is the turn ratio of a first transformer associated with the firstcharge converter circuit. The output of the first charge convertercircuit would be inputted into the second charge converter circuit,wherein the second charge converter circuit is adapted to increase ordecrease the desired output voltage Vout2 by a second voltage stepVs2=Vsup*M2/N2, where Vsup is the supply voltage and M2/N2 is the turnratio of a second transformer associated with the second chargeconverter circuit. The supply voltage Vsup can be the same for both thefirst and second charge converter circuits. However, in otherembodiments, the first and second charge converter circuits can haverespective supply voltages Vsup which are different. The stepgranularity of the cascaded converters would be determined by the lowestvalued voltage step between the first voltage step Vs1 and the secondvoltage step Vs2. In yet another embodiment, the charge convertercircuit can be arranged to be comprised of a push-pull circuit insteadof a transformer based circuit.

While particular embodiments of the invention have been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

1. (canceled)
 2. A DC-DC converter circuit, comprising: a voltage sourcewhich provides a voltage Vout1 at a first node; an output filtercapacitor having an associated output voltage Vout2; and at least onecharge converter circuit, comprising: a transformer having at least oneprimary winding and at least two secondary windings; a primary circuithaving at least two primary switches connected between a supply voltageand respective sides of the primary winding; a secondary circuit havingat least two secondary switches, wherein a first pair of said secondaryswitches are connected to respective ones of the secondary windingsbetween the first node and the output filter capacitor; and a controlunit arranged to: receive a control signal, wherein the control signalrepresents a desired output voltage Vout2; and provide output controlsignals to the primary and secondary switches including to activate anddeactivate the primary and secondary switches as needed to obtain thedesired output voltage Vout2, including to deactivate the primaryswitches and to activate the secondary switches when Vout1=Vout2, suchthat magnetic flux in the transformer is substantially zero.
 3. TheDC-DC converter circuit of claim 2, said control unit arranged tooperate said primary and secondary switches as needed to dischargeenergy from said output filter capacitor and transfer the dischargedenergy into said primary circuit when said control signal indicates thatVout2 is to decrease.
 4. The DC-DC converter circuit of claim 2, saidcontrol unit arranged to operate said primary and secondary switches asneeded to charge said output filter capacitor when said control signalindicates that Vout2 is to increase.
 5. The DC-DC converter circuit ofclaim 2, wherein said primary switches and secondary switches areconfigured to be pre-biased to reduce the switching time required toactivate and deactivate said primary or secondary switches.
 6. DC-DCconverter circuit of claim 2, wherein said at least one charge convertercircuit is comprised of a first charge converter circuit and a secondcharge converter circuit arranged in a cascade configuration, whereineither of said first or second charge converter circuits can be a boostconverter circuit or a buck-boost converter circuit.
 7. The DC-DCconverter circuit of claim 2, wherein said primary circuit is comprisedof four primary switches and said secondary circuit is comprised of twosecondary switches.
 8. A multilevel DC-DC converter, comprising: avoltage source which provides a voltage Vout1 at a first node; an outputfilter capacitor having an associated output voltage Vout2; and at leastone charge converter circuit, comprising: a transformer having at leastone primary winding and at least two secondary windings; a primarycircuit having at least two primary switches connected between a supplyvoltage and respective sides of said primary winding; a secondarycircuit having at least two secondary switches, wherein a first pair ofsaid secondary switches are connected to respective ones of thesecondary windings between the first node and the output filtercapacitor; and a control unit arranged to: receive a control signal,wherein the control signal represents a desired output voltage Vout2;and provide output control signals to the primary and secondary switchesincluding to activate and deactivate the primary switches and toactivate and deactivate the secondary switches over a time period whenVout2>Vout1 or when Vout2<Vout1, such that the voltage level of saidoutput voltage Vout2 is substantially similar to the desired outputvoltage represented by the control signal.
 9. The multilevel DC-DCconverter of claim 8, wherein the control unit is arranged to increaseor decrease said output voltage by a voltage step Vs when indicated bysaid control signal, wherein said voltage step Vs=Vsup*M/N, wherein Vsupis said supply voltage and M/N is a turn ratio of said transformer. 10.The multilevel DC-DC converter of claim 8, said control unit arranged tooperate said primary and secondary switches as needed to dischargeenergy from said output filter capacitor and transfer the dischargedenergy into said primary circuit when said control signal indicates thatVout2 is to decrease.
 11. The multilevel DC-DC converter of claim 8,said control unit arranged to operate said primary and secondaryswitches as needed to charge said output filter capacitor when saidcontrol signal indicates that Vout2 is to increase.
 12. The multi DC-DCconverter of claim 8, wherein said primary switches and secondaryswitches are configured to be pre-biased to reduce the switching timerequired to activate and deactivate said primary or secondary switches.13. The multilevel DC-DC converter of claim 8, wherein said at least onecharge converter circuit is comprised of a first charge convertercircuit and a second charge converter circuit arranged in a cascadeconfiguration, wherein either of said first or second charge convertercircuits can be a boost converter circuit or a buck-boost convertercircuit.
 14. The multilevel DC-DC converter of claim 8, wherein saidsecondary circuit is comprised of four secondary switches, wherein asecond pair of said secondary switches is connected to said secondarywindings opposite said first pair.
 15. A multilevel DC-DC converter,comprising: a voltage source which provides a voltage Vout1 at a firstnode; an output filter capacitor having an associated output voltageVout2; and at least one charge converter circuit, comprising: atransformer having at least one primary winding and at least twosecondary windings; a primary circuit having at least two primaryswitches connected between a supply voltage and respective sides of theprimary winding; a secondary circuit having at least two secondaryswitches, wherein a first pair of the secondary switches are connectedto respective ones of the secondary windings between the first node andthe output filter capacitor; and a control unit arranged to receive acontrol signal which represents a desired output voltage Vout2 and toprovide output control signals to the primary and secondary switchessuch that the output control signals activate and deactivate saidprimary and secondary switches as needed to obtain the desired outputvoltage Vout2, including to increase said output voltage Vout2 by avoltage step Vs when indicated by the control signal, wherein thevoltage step Vs=Vsup*M/N, wherein Vsup is the supply voltage and M/N isa turn ratio of the transformer.
 16. The multilevel DC-DC converter ofclaim 15, wherein said primary switches and secondary switches areconfigured to be pre-biased to reduce a switching time required toactivate and deactivate said primary or secondary switches.
 17. Themultilevel DC-DC converter of claim 15, said control unit arranged tooperate said primary and secondary switches as needed to dischargeenergy from said output filter capacitor and transfer the dischargedenergy into said primary circuit when said control signal indicates thatVout2 is to decrease.
 18. The multilevel DC-DC converter of claim 15,said control unit arranged to operate said primary and secondaryswitches as needed to charge said output filter capacitor when saidcontrol signal indicates that Vout2 is to increase.
 19. The multilevelDC-DC converter of claim 15, wherein said primary switches and secondaryswitches are configured to be pre-biased to reduce the switching timerequired to activate and deactivate said primary or secondary switches.20. The multilevel DC-DC converter of claim 15, wherein said at leastone charge converter circuit is comprised of a first charge convertercircuit and a second charge converter circuit arranged in a cascadeconfiguration, wherein either of said first or second charge convertercircuits can be a boost converter circuit or a buck-boost convertercircuit.
 21. The multilevel DC-DC converter of claim 15, wherein saidprimary circuit is comprised of four primary switches and said secondarycircuit is comprised of two secondary switches.